One type of a conventional EE-PROM mainly comprises memory cells arranged in a matrix pattern, a sense amplifier circuit for producing a signal dependent on the content of a memory cell selected from the matrix memory cells, a reference voltage producing circuit for producing a reference signal, and a differential amplifier for comparing the signal dependent on the content of the selected memory cell and the reference signal and amplifying a compared difference between the signals.
In operation of the reading mode, a reading voltage is applied to a gate electrode of a transistor for the selected memory cell. Then, the transistor is turned on where the memory cell has been programmed, while the transistor is turned off where the memory cell has been erased. As a result, a voltage V.sub.ON which is lower than the reference signal is supplied from the sense amplifier circuit in the former state, while a voltage V.sub.OFF which is higher than the reference signal is supplied from the sense amplifier circuit in the latter state. Consequently, a signal "L" is supplied from the differential amplifier in the former state, while a signal "H" is supplied therefrom in the latter state. Accordingly, stored information can be read from the selected memory cell. The construction and operation of the conventional EE-PROM will be described in detail latter.
According to the conventional EE-PROM, however, there is a disadvantage that a speed of reading a content of a selected memory cell is not so fast as expected for the reason why the time in which a reference signal level is set to be a reference value is difficult to be shortened. The reasons thereof will be explained in more detail later.